It is known to be convenient, for use in the injection of resins, to utilize an injection nozzle having a needle valve adapted to be retracted by the pressure of the resin to open the nozzle as the injection is commenced, the pressure being applied to a tapered portion of the needle, the needle being further adapted to be returned by spring force, when the injection is completed, e.g. to close the nozzle by a force exerted by a piston spring, associated with a piston, provided on the periphery of the nozzle.
This type of injection nozzle has been found, however, to be inconvenient in some aspects in that its piston relies entirely upon a pressure differential between the pressure of the resin and the ambient atmospheric pressure. Thus, the piston receive the ambient atmospheric pressure directly on one face, and is also subjected to the high temperature of the resin which may be 150 to 350.degree. C., and with repeated application of a pressure between 0 and 120 Kg/cm.sup.2. Accordingly, a leakage of resin tends to occur around the piston, no matter how carefully and precisely the piston may be machined. This tendency to leak cannot be avoided, even by the provision of piston rings. Once the leakage takes place, the resin tend to gum up the parts adjacent to the nozzle, e.g. a nozzle heater, electric wires for the heater and a locator ring, and becomes accummulated thereon. Considerable time and labor are required to remove such accumulations of resin. For these reasons, injection nozzles having a piston relying upon the pressure differential between the resin and atmospheric pressure are not used very widely.
Another conventional injection nozzle called the "slide head type" has been found also inconvenient in that it requires the continuous operation of an injection bed, while still another conventional nozzle called the "shut-off type" disadvantageously necessitates a complicated structure for actuating a hydraulic piston for effecting the opening and closing of the nozzle. Both the "slide head type" and the "shut-off type" have a common drawback of leakage of resin, attributable to a direct contact of the piston with the atmosphere.
To overcome these drawbacks, there has recently been proposed a needle valve making use of a pressure differential of the resin and relying upon Pascal's law. However, the leakage of resin of the spring section can never be acceptably reduced even in this type of injection nozzle.